The dark reaction is independent of light. This reaction is purely enzy­matic and is carried out in the stoma portion of the chloroplast. Ribulose-1, 5-diphosphate (RuDP), a pentose phosphate present in plant cells, acts as the initial acceptor of CO2 and changes thereby into a very unstable C6. The latter is converted into 3-phosphoglyceric acid (3-PGA), which is transferred to 3-phosphoglyceraldehyde. For this reaction, ATP and NADPH2 (produced in the light reaction) are necessary as cofactors. Three molecules of RuDP combine with three molecules of CO2 to give rise to six molecules of PGA. Three molecules of RuDP utilized initially as CO2 acceptors are regenerated by five molecules of phosphoglycer — aldehyde through different intermediates like xylulose-5-phosphate and ribulose-5-phosphate. The only molecule of phosphoglyceraldehyde is converted into fructose-1,6-diphosphate, which may be transformed into sucrose and starch through other reactions.